1. Field of the Invention
The present invention relates to a compressor for a cooling apparatus, and more particularly, to a method for controlling a reciprocating compressor having variable capacity, which is capable of varying a rotational speed thereof.
2. Description of the Related Art
Generally, a compressor is used for compressing a refrigerant at a high temperature and a high pressure, which circulates in an apparatus for generating cool air. There are many kinds of compressors such as a reciprocating compressor, a rotary compressor, a brushless direct current (BLDC) compressor and an inverter type compressor.
A single-phase induction motor is typically used in the reciprocating compressor and the rotary compressor. As shown in FIG. 1, a frequency of 50 or 60 Hz is used to start the compressors. In the above mentioned compressors which use the single-phase induction motor, only an on/off operation of the compressors is controlled without any variation of a rotational speed thereof.
Meanwhile, in an reciprocating compressor having variable capacity, the rotational speed of the compressor can be controlled by varying a supplied voltage or frequency. Therefore, a flow rate of the refrigerant is facilely adjusted by the variation of the rotational speed of the compressor, whereby the cooling apparatus itself can be directly controlled. FIG. 3 shows a structure of a control board in connection with the reciprocating compressor having variable capacity.
In the FIG. 3, a motor 1 of the reciprocating compressor having variable capacity is driven by an inverter 3 operated by an output signal from a programmable array logic (PAL) 2. The PAL 2 is controlled by a microcomputer 10. The rotational speed of the motor 1 is input to the microcomputer 10 through a back emf sensing part 4. Therefore, the microcomputer 10 precisely controls the speed of the motor 1 referring to the input data of the speed of the motor 1.
The microcomputer 10 comprises a mode selecting part 11, a commutator 12 which receives a signal from the mode selecting part 11 and outputs a commutation signal to the PAL 2, a step signal generating part 13 which generates a step signal and transmits the step signal to the mode selecting part 11, a digital-phase shifter 14 which converts the signal of the rotational speed from the back emf sensing part 4 into a correspondent digital phase shifter signal and transmits the digital phase shifter signal to the mode selecting part 11, and a PWM pulse part 15 which applies a pulse width modulation (PWM) signal to the PAL 2.
However, in the above mentioned reciprocating compressor having variable capacity, there is a problem that, since an angular velocity of a shaft of the compressor is much lower than that of the conventional compressor using the single-phase induction motor, a lubricating oil is not sufficiently supplied to a sliding portion of the compressor when the compressor is operated at a minimum speed, whereby a reliability of parts of the compressor is lowered.
To overcome the above problem, there has been suggested a method which starts a compressor at a frequency of 58 Hz. However, in the method, there is a problem.
In addition, in the conventional reciprocating compressor having variable capacity, there is a problem that a noise and vibration due to a sudden change of the rotational speed of the compressor is generated when changing the speed from the maximum to the minimum or reversely.